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– Through identification of applications, it is apparent that there are three
types of adhesive applications, (thin bond, gap filling and adhesive
based mortars/grouts). These can be linked to actual applications, by
way of consideration of the substrate materials. The choice of adhesive
is a function of the substrates and the process by which the adhesive is
incorporated in the final product, component or assembly.
– The key factors to be considered in order to establish a link between
adhesive performance and timber design codes are duration of load effects
and service class effects, which act in combination to define the
design behaviour model for both serviceability and ultimate limit states.
– A limited range of experimental evidence for this is already available,
with certain confirmatory of practical importance, and with certain adhesive
classes.
35-18-1 C Bengtsson, B Källander
Creep testing wood adhesives for structural use
Introduction and background
Lack of approval procedures for adhesives is hampering, the development
of wood products. There is an urgent need for fast and reliable approval
procedures for new wood adhesive types, new gluing processes and new
glued wood based products. SP is conducting research on test procedures
to determine creep properties of wood adhesives for structural purposes.
The research has been aimed at developing fast and reliable methods for
approval of structural adhesives. The work is carried out within the
framework of CEN / TC193 ) / SCI / WG4 with financial support from the
Swedish Wood Association.
Existing test methods for adhesives for structural use have been developed
for aminoplastic and phenolic adhesives such as Phenol Resorcinol
Formaldehyde (PRF) and Urea Formaldehyde (UF). These adhesives show
very little or no creep and hence no test procedures for creep deformation
or creep rupture testing of structural wood adhesives have been established.
New adhesives such as PolyUrethane (PU) and Emulsified Polymer
Isocyanates (EPI) show certain creep tendencies. In order to approve such
adhesives, the amount of creep must be determined and related to demands
of the finished products.
As for all accelerated test methods, it is crucial that the developed test
methods reflect the expected failure modes in the climates that the glued
structure will meet in practice. An important aspect regarding creep is the
glass transition temperature of the adhesive. An accelerated test at high
temperature and moisture could result in failures that never would occur at
lower temperatures. There is also the question of whether a test for wood,
adhesive or glulines is developed. The combined effects of temperature
and moisture on the adhesive as well as the wood properties will limit the
possibilities to accelerate creep tests if failure modes are to maintain unaltered.
A serious difficulty regarding accelerated creep test methods for adhesives
is the question on how to set the requirements for the tests. Without
long term experience of the actual adhesive types in real practice, we need
to establish initial requirements based on theoretical assumptions. SP suggests
that such initial requirements should be based on the load levels,
climates and time spans set in Eurocode 5.
Although the developed test methods should be as fast and cheap as
possible in order to simplify the introduction of new products, it is crucial
that the developed test methods produce safe and reliable results. One important
aspect is then that the limited experience of the test methods and
the lack of established requirements make it important that the test methods
produce results that can be re-evaluated when new experience is gathered.
Conclusions and recommendations
The results have shown that both methods European 3535 and European
4680 can differentiate between adhesives with different creep properties.
Possibly both methods can in the future be used in approval of PU adhesives
for structural purposes.
The tests with European EN4680 show large variations in results of the
same 1 component PU
adhesive type, both between individual test samples as well as between
different batches. Similar variations are likely to influence also results of
the European EN 3535 test method. The effects of such variations on the
testing and approval procedure need to be evaluated.
Conclusions regarding the European EN 3535 method
The European RN 3535 method gives a fail / pass test result within an
eight weeks period after production of test samples. The test result depends
on time to failure for the weakest of 72 glulines surfaces. The complicated
sample design and non centred load application makes the method
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.41